The present disclosure generally relates to document processing devices and methods for operating such devices. More specifically, the present disclosure relates to methods and systems for extending roll life in feed nips of a friction retard feeder.
Friction retard feeders are subsystems of a document processing system that are typically used to initiate reliable transport of a sheet from a tray to the remainder of the document processing system while preventing the transport of more than one sheet simultaneously. For example, in the case of a printer, scanner or other similar device, a friction retard feeder can be used to remove the top sheet of paper or other media from a paper tray so that it can be utilized by the remainder of the device while preventing underlying sheets from being transported. This reduces the possibility of jamming the device during operation.
Friction retard feeders are typically either active or semi-active in nature. An active friction retard feeder, such as is shown in FIG. 1A, includes a nudger roll 105, a feed roll 110 and a retard roll 115 having a reverse direction rotating shaft and a slip clutch 120. The nudger roll 105 is used to drive the top sheet on a stack of sheets to the feed roll 110. The feed roll 110 is rotated such that a media to be transported, such as paper, that contacts the feed roll is transported in a process direction. The retard roll 115 is connected to a drive shaft that causes the retard roll to rotate in a direction that would transport a sheet in a direction opposite to the process direction when active. However, a slip clutch 120 coupled to the drive shaft disengages the retard roll 115 from the drive shaft when a torque sufficient to overcome the torque of the slip clutch is present. As such, when the retard roll 115 contacts either the feed roll 110 directly or if only one sheet enters the feed nip (i.e., is between the feed roll and the retard roll) as is shown in FIG. 1B, the slip clutch 120 slips (i.e., the torque of the slip clutch is overcome) and the retard roll rotates in the process direction. However, if more than one sheet enters the feed nip as shown in FIG. 1C, the sheet-to-sheet friction force on the bottom sheet(s) is insufficient to overcome the torque of the slip clutch 120 and the retard roll 115 rotates opposite to the process direction (i.e., in a non-process direction).
The active friction retard feeder operates on a differential friction principle by employing a feed roll 110 that has a high coefficient of friction with respect to the media and a retard roll 115 that has a coefficient of friction with respect to the media that is lower than that of the feed roll, but higher than the coefficient of friction between two sheets. As such, the feed roll 110 transports the top sheet in the process direction because it has a high coefficient of friction with the sheet. Moreover, the sheet has a coefficient of friction (due to the force imparted by the feed roll 110) to overcome the torque supplied by the slip clutch 120 causing the retard roll 115 to be driven by the feed roll. If more than one sheet is drawn from the stack, the sheets other than the top sheet contact the retard roll 115 which prevents transport in the process direction because the torque of the slip clutch 120 is not overcome by the friction between the two sheets. As a result, only a single sheet, the top sheet, is transported in a process direction at a time.
A semi-active friction retard feeder, as shown in FIG. 2A, also includes a nudger roll 205, a feed roll 210 and a retard roll 215 having a slip clutch 220. Each of the nudger roll 205, feed roll 210 and retard roll 215 operate in a substantially similar manner to the corresponding components of the active friction retard feeder. However, the retard roll 215 of the semi-active friction retard feeder is not driven by a drive shaft. Rather, the retard roll 215 is merely rotatably mounted on a shaft. As such, when the torque of the slip clutch 220 is not overcome (i.e., when more than one sheet is present), the retard roll 215 merely remains immobile instead of actively driving the sheets other than the top sheet in the non-process direction.
An alternate friction retard feeder (such as is shown in FIG. 3) includes a retard pad 315 instead of a retard roll, such as 115 or 215. In order to prevent feeding of more than one sheet, the retard pad 315 must have a lower coefficient of friction with respect to a sheet than the feed roll 310, but a higher coefficient of friction with respect to a sheet than the coefficient of friction between two sheets.
Friction retard feeders are commonly incorporated into document processing systems that serve office and low end production markets because they have a low unit manufacturing cost and provide reliable feeding performance. One disadvantage of such feeders is that the feed and retard rolls wear out and need to be replaced frequently because of the shear forces on the elastomeric rolls due to drag forces and the force required to overcome the slip clutch torque. For document processing devices in low volume markets, this can be an acceptable limitation because of the limited amount of sheets which are passed through the feed nip. However, this disadvantage has restricted the use of friction retard feeders in higher volume markets.
Significant research has been performed in order to develop feed rolls that have a longer wear life. However, the effect of such research is limited by the fact that materials must also resist contamination from, for example, coated media, such as photo paper.